Technologies for personal and vehicular navigation are rapidly developing due to the availability of Global Navigation Satellite Systems (GNSS) such as the United States' Global Positioning System (GPS), the Russian GLONASS system, and the European Galileo system. These systems however are designed for environments where a clear line of sight (LOS) exists between the user receiver and the GNSS satellites. Using trilateration methods, a user can convert the ranging measurement obtained from the LOS signal into an estimate of the user's position.
Indoor and urban canyon navigation presents two main challenges for GNSS: low signal strength and severe multipath interference. Assisted GPS (AGPS) and Ultra-Tight Coupling (UTC) are current methods for tracking weak signals in the GNSS receiver operating environment. Although there are several existing techniques for mitigating multipath interference, these known techniques are not effective to adequately mitigate the multipath in the indoor and urban environments. Multipath interference exists when a receiver receives signals reflected from nearby terrain or structures, such as buildings or walls. The received multipath signal is commonly referred to as a non-line of sight (NLOS) signal. Multipath signals always arrive “late” compared to the LOS signal, thus creating an error in the measured range and corrupting the user position estimate. This problem is especially acute indoors and in urban canyons, due to multiple reflection-generating objects which surround a user (building walls, furniture, cars, etc.).
There are several known methods to mitigate multipath, but these methods help only in relatively benign multipath environments. For example, if the strength of muitipath signals is not large compared to the strength of the LOS signal, and multipath delays are not too small, narrow correlators, strobe correlators and similar methods may be used to effectively isolate or remove the multipath signals. However, these techniques are effective only if the multipath delays are on the order of or larger than the inverse signal bandwidth, e.g., at least 0.1 chip length, which is 30 meters for GPS civilian signal. However, in the indoor and urban environment, there are many multipath-generating surfaces at distances smaller than 30 meters, which makes these techniques not effective for those environments.
Another known technique for multipath mitigation is called “Multipath Estimation Delay Lock Loop” (MEDLL) from NovaTel, Inc. in Calgary Alberta, Canada. A MEDLL receiver has many correlators which integrate the satellite signal at different delays (compared to typically three correlators for a traditional receiver) against the known code of the transmitted signal. The result is a profile of a GNSS satellite signal's correlation with the code replica, sampled at an array of points. LOS and all multipath components contribute to this profile, forming a complex signature. The MEDLL can discriminate the individual signal components, thus can be used to isolate LOS from multipath. This method is effective if there are few dominant signal components, and if one of the dominant components is the LOS signal. If there are many multipath components and/or if LOS is not present or weak, the MEDLL receiver does not yield a reliable LOS measurement.